1. Field of the Invention
The present invention relates to an image bearing member, an image forming apparatus and a process cartridge.
2. Discussion of the Background
Recently, organic image bearing members (organic photoconductors (OPC)) have been superseding inorganic image bearing members in the application to photocopiers, facsimile machines, laser printers and their multi-functional machines due to good performance and various other merits. Specific reasons include optical characteristics, for example, the width of the optical absorption wavelength range and the amount of optical absorption electric characteristics, for example, high sensitivity and stable charging characteristics, the wide range of selection for materials, easy manufacturing, low cost, and non-toxicity.
On the other hand the diameter of an image bearing member has been recently reduced in accordance with reduction in the size of an image forming apparatus. On top of that, high speed and maintenance-free performance have been required. Therefore, an image bearing member having high durability is demanded.
An organic image bearing member includes a surface layer mainly formed of a low molecular weight charge transport material and an inactive polymer. Such an image bearing member is soft in general. This leads to a problem that, while an organic image bearing member is repeatedly used in the electrophotographic process, the organic image bearing member easily abrades away due to the mechanical burden on the organic image bearing member received from a developing system and a cleaning system. In addition, toner particles have been reduced in size to improve the quality of images, which requires improvement of the cleaning property and leads to an increase in the hardness of the rubber for use in a cleaning blade and the contact pressure thereof to an image bearing member. This is one of the causes which accelerates the abrasion of an image bearing member. Such abrasion of an image bearing member degrades electric characteristics thereof, for example, sensitivity and chargeability. Thereby, the image density deteriorates and background fouling occurs, resulting in abnormal images. In addition, scars locally made on an image bearing member due to abrasion invite poor cleaning performance, which leads to fouling in a streak manner on an image. In the current status, abrasion and scars on an image bearing member are rate controlling factors for replacement thereof.
Therefore, it is inevitable to decrease the abraded quantity to improve the durability of an organic image bearing member. This is an imminent issue to be solved in this area.
As technologies for improving an anti-abrasion property of a photosensitive layer, for example, (1) unexamined published Japanese patent application No. (hereinafter referred to as JOP) S56-48637 describes a surface layer containing a curing binder; (2) JOP S64-1728 describes a technology in which a polymer type charge transport material is used; and (3) JOP H04-281461 describes a surface layer in which an inorganic filler is dispersed.
In the method described in (1), due to insufficient compatibility with the charge transport material and the presence of impurities, for example, a polymerization initiator and unreacted remaining groups, the remaining voltage tends to rise and the image density tends to deteriorate.
In addition, the methods described in (2) and (3) possibly improve the anti-abrasion property of an organic image bearing member but do not improve the anti-abrasion property to a level desired therefor.
Furthermore, in the method described in (3), the remaining voltage easily rises due to the trapped charge present on the surface of an inorganic filler, which leads to the tendency of deterioration of the image density.
These technologies described in (1) to (3) do not sufficiently provide an organic image bearing member with the total durability desired therefor including electric durability and mechanical durability.
Furthermore, Japanese patent No. (hereinafter referred to as JP) 3262488 describes an image bearing member containing a polyfunctional curing type acrylate monomer to improve the anti-abrasion property and anti-damage property described in (1). In this image bearing member, the protective layer, which is provided on the photosensitive layer, contains a polyfunctional curing type acrylate monomer. However, there is a description that only the protective layer can contain a charge transport material. Furthermore, when a surface layer simply contains a charge transport material having a low molecular weight, a problem of the compatibility with the cured material arises, which causes problems such that the charge transport material having a low molecular weight precipitates, white turbidity phenomenon occurs and the mechanical strength deteriorates.
Furthermore, with regard to this image bearing member, a monomer reacts while a polymeric binder resin is contained, which causes insufficient curing and a problem of the compatibility between the cured material and the binder resin. As a result, the surface tends to be convexo-concave due to phase separation during curing and cause poor cleaning performance.
As an anti-abrasion technology for a photosensitive layer superseding these, JP 3194392 describes a method in which a charge transport layer is formed by application of a liquid containing a monomer having a carbon-carbon double bond (C═C), a charge transport material having a carbon-carbon double bond (C═C), and a binder resin. The binder resin contains a material having a carbon-carbon double bond (C═C) and reactive with the charge transport material and a material not having a carbon-carbon double bond (C═C) and non-reactive with the charge transport material. This image bearing member has a good combination of anti-abrasion property and electric characteristics and draws attention. However, when a non-reactive binder resin is used, the binder resin and a cured material formed through the reaction between the monomer and the charge transport material are not sufficiently compatible, causing phase separation. This tends to lead to the formation of a convexo-concave surface during cross-linking and result in bad cleaning performance. Additionally the binder resin prevents the curing of the monomer. The specifically described monomers are divalent monomers so that the obtained cross linking density is not sufficient and the anti-abrasion property is not satisfactory. Furthermore, even when a reactive binder resin is used since the monomer and the binder resin have a low number of functional groups, it is difficult to have a good combination of the content of linked charge transport materials and cross linking density and obtain sufficient electric characteristics and anti-abrasion properties.
JOP 2004-66425 describes a photosensitive layer containing a compound cured from a positive hole carrier transport compound having at least two chain-reaction polymerizable functional groups in one molecule.
However, since this photosensitive layer contains bulky positive hole carrier transport compounds having at least two chain reaction polymerizable functional groups, distortion may occur in the cured material, which invites a high internal stress and leads to the roughness of the surface layer and the occurrence of cracking over tire. As a result, the photosensitive layer does not have sufficient durability.
Currently these image bearing members of the background art, having a cross linking photosensitive layer in which the charge transport structure is chemically linked, are not sufficient in terms of comprehensive characteristics.
To improve the current situation, the present inventors describe a technology in JOP 2004-302451 of curing a radical polymerizable monomer having at least three functional groups which does not have a charge transport structure and a radical polymerizable compound having one functional group which has a charge transport structure. However, in the technology an irradiating device is used to cure these radical polymerizable monomers for forming the surface layer so that the characteristics of the image bearing member tend to vary depending thereon.